Bean Leaf Beetle

Encyclopedia Article

The bean leaf beetle, Cerotoma trifurcata, is a member of the leaf beetle family Chrysomelidae and the order Coleoptera. This leaf beetle was first described in 1771. Bean leaf beetle adults can feed on soybean foliage or directly on the pods, and beetles can vector bean pod mottle virus. The majority of economic injury occurs in high-value soybeans, such as food-grade soybeans or soybeans for seed.

Identification

Adult: The adult is oval-shaped and typically dark yellow with black markings (Photo 1); however, adults can be green, yellow, orange, and red. Adults are about 5 mm (1/5 inch) in length and have four, large, quadrangular, black markings on the elytra (forewings). Occasionally, these four rectangular marks are reduced to two, or they may be completely absent. The most constant identifying character for this beetle is the presence of a black triangle at the “neck" region.

bean leaf beetle
Photo 1. Bean leaf beetle with typical coloration and markings. Photo Marlin E. Rice.

Egg: The egg is orange and spindle-shaped, only a few millimeters long and wide. Eggs are laid in the soil, usually around the base of soybean plants.

Larva: The larva is white and cylindrical with a dark brown head and brown sclerite (plate) on the top side of the last abdominal segment (Photo 2). Larvae occur in the soil and are approximately 3/8 inch long at maturity.

bean l
Photo 2. First instar bean leaf beetles. The white globules are the fat bodies within the insect’s body cavity. The dark brown area along the midline of the body is food passing through the gut. Photo by Jeff Bradshaw.

Pupa: The pupa is white, immobile, and slightly resembles the shape of the adult beetle.

Biology and Ecology

Distribution: Bean leaf beetle is native to North America and may be common anywhere soybeans are grown. The bean leaf beetle is common throughout the northcentral United States and has been recorded from Illinois, Indiana, Iowa, Kansas, Kentucky, Michigan, Minnesota, Missouri, Nebraska, Ohio, South Dakota, Wisconsin, and the Canadian provinces of Manitoba and Ontario.

Host Plants: Bean leaf beetle has a large host range that mostly includes legumes such as soybean, green bean, and clover; however, it will occasionally feed on stinging nettle and cucurbits such as pumpkin and cucumber (Photo 3).

bean leaf beetle on pumpkin
Photo 3. Bean leaf beetle feeding on pumpkin (var. "Magic Lantern"). Photo by Robert Koch.

Life Cycle: In the spring, adult beetles emerge from their overwintering habitat and migrate to available host plants to mate. They are commonly found in alfalfa and other legumes such as tick trefoil and various clovers in late April or early May. As the season progresses, bean leaf beetles are found on more preferred hosts (e.g., soybean or green bean). Note that although these first beetles begin to appear well before soybean emergence, peak abundance can coincide with the emergence of soybean (depending on the date the soybean was planted).

Adult beetles that colonize crops such as soybean will deposit eggs in the upper two inches of soil near the base of plants. A female usually lays 125-250 eggs in a lifetime. Eggs will hatch in about one week, depending on temperature, and larvae will feed on soybean roots and root nodules. Larvae feed for approximately three weeks before pupating in the soil. Larval survival requires highly organic soils (approximately 66% organic matter) and is inversely proportional to clay content. During their time in the soil, bean leaf beetles are most susceptible to heavy rainfall. In Iowa, the time required for the development of the first generation from egg-laying to adult emergence is approximately 25-40 days.

The number of generations bean leaf beetle has every year is somewhat dependent on latitude. This beetle is multivoltine (three generations per year) in the southeastern U.S.; bivoltine (two generations per year) in Iowa and Illinois (Figure 1); uni- or bivoltine in Wisconsin; and univoltine (one generation per year) in Ontario, Canada. Approximately 80% of adults overwinter in leaf debris in woodlots; however, some do overwinter is soybean field residue. Survival for overwintering beetles is governed by the number of days accumulated below the temperature of 14oF. Studies by Lam and Pedigo found that >50 percent of bean leaf beetles can survive for hundreds of hours at 23oF; however, most beetles died by 15 minutes at 14oF. In southern latitudes, it is not clear what key factors are responsible for mortality during the winter; however, the weather is thought to be important.

bean leaf beetle life cycle
Figure 1. Bivoltine life cycle of bean leaf beetle observed in Iowa.

Dispersion and Dispersal: On average, bean leaf beetles have the physiological capacity for flying short distances (<167 ft) and populations at the field scale are highly aggregated. The spatial relationship between the first and second generations are highly correlated to each other with the second generation showing the greatest aggregation pattern in soybean fields.

Dispersion patterns and flight potential of bean leaf beetles are both related and relevant for understanding how these beetles affect soybean. They are related because short flights within each generation increase the likelihood of establishing aggregated populations. Because bean leaf beetles generally fly short distances and establish aggregated populations within fields, there is potential for site-specific management. Additionally, if this same pattern and flight potential is true regardless of population size, local beetle populations should be predictable from year to year. Lastly, short flights and highly aggregated populations should result in localized infections of diseases that might be transmitted by bean leaf beetles (e.g., bean pod mottle virus).

Plant Injury

Generally, bean leaf beetles exist as either root-feeding larvae or leaf- and pod-feeding adults. Depending on the plant stage, the amount of feeding activity or injury can have a profound impact on soybean growth and development.

Although bean leaf beetle larvae feed on soybean roots and nodules (Photo 4), there is no evidence that larvae are economically significant. The relationship between adult injury of soybean and yield is better understood. Bean leaf beetles feed for about 21 days, chewing holes in leaves (Photo 5) or pods (Photo 6). Soybeans are well-known to compensate for defoliation, but heavy infestations can cause yield loss, especially on small plants. When feeding on pods, bean leaf beetles occasionally damage the seed. A primary concern with bean leaf beetle is the transmission of bean pod mottle virus or the secondary infection of fungi (e.g., Diaporthe spp. and Phomopsis spp.) into the pods due to feeding.

indirect injury of bean leaf beetle larva to soybean roots
Photo 4. Indirect injury by bean leaf beetle larva to roots. Photo by Jeff Bradshaw. 

indirect injury of bean leaf beetle
Photo 5. Indirect injury to soybean leaves. Photo by Marlin E. Rice. 

direct injury of bean leaf beetle to soybean pods
Photo 6. Direct injury by bean leaf beetle to soybean pods. Photo by Marlin E. Rice. 

Bean pod mottle virus: Bean leaf beetle is the primary vector for bean pod mottle virus. This virus is the most prevalent soybean virus in the North Central states. Possible sources of primary inoculum include soybean seed, overwintering bean leaf beetles, and perennial legumes.

On soybeans, bean pod mottle virus may cause severe systemic mottling with a puckering of leaflets (Photo 7) and mottling of pods and seed coats; however, symptomatic response varies by soybean variety, soybean stage at inoculation, and planting date. When present, bean pod mottle virus-symptoms can be difficult to distinguish from other viral symptoms (e.g., soybean mosaic virus) and may be related to the development of green stem syndrome (Photo 8). Additionally, the foliar symptoms of bean pod mottle virus may be masked at high temperatures, and cool temperatures favor development of symptoms.

leaf motting caused by bean pod mottle virus
Photo 7. Leaf mottling caused by bean pod mottle virus. Photo by Marlin E. Rice. 

quantal injury of bean leaf beetle resulting in green stem
Photo 8. Quantal injury resulting in green stem.

Yield may be reduced in bean pod mottle virus-infected plants due to reduced seed size and pod set, which is most severe when soybeans are infected as seedlings. Seed-infection either does not occur or occurs at a very low infection-rate, with virus usually present in the seed coat. Aside from yield, farmers can lose money from bean leaf beetles vectoring bean pod mottle virus when seed coat quality is reduced on food-grade soybeans (Photo 9). However, for soybean grain where seed-coat quality is not a concern, this virus is of lesser concern because bean pod mottle virus does not affect the color of a soybean seed beneath the seed coat.

seed discoloration caused by bean pod mottle virus
Photo 9. Seed discoloration caused by bean pod mottle virus.

Risk Factors

The first-emerging soybean fields each year are likely at greater risk for early-season infestations by bean leaf beetle. Soybean fields with a recent history of large populations of bean leaf beetles may be at risk for yield and quality reductions from both bean pod mottle virus and bean leaf beetle. The primary management concern is in food-grade soybeans or soybeans grown for seed. Soybean grain is subject to stringent standards of seed coat quality when sold for food.

Scouting

Because overwintering beetle populations vary year to year, it is important to scout for this pest early in the season. There are several methods that exist depending on when sampling occurs during the growing season. Consider scouting soybean fields, especially if:

  1. Soybean is planted near alfalfa fields or if the field has the first-emerging soybean in the area. Overwintering adults are strongly attracted to soybean and will move into fields with emerging plants.
  2. Fields are planted to food-grade soybeans or are seed fields where reductions in seed quality can be significant.
  3. Fields have a history of high beetle populations or bean pod mottle virus.

Early-season – calculate beetles per plant: Randomly select five sampling sites across the field. At each site, slowly walk down 15-20 feet of row and count beetles and plants. Do not disturb the plants but get close enough to see underneath leaves and in the growing point. Also, note whether the growing point has been damaged or killed. Refer to Table 1 for guidance on economic thresholds for early-season soybean sampling.

Table 1. Economic threshold (beetles/plant) for early-season scouting (VE to V3). Table modified from University of Nebraska and Purdue Extension.

Crop Value ($/bu) Control Cost ($/acre)
6.00 8.00 10.00 12.00 16.00
5.00 3 4 5 6 8
6.00 3 4 5 5 7
7.00 2 3 4 4 6
8.00 2 3 3 4 6

Mid-season – estimate defoliation and insect density: Determine percentage defoliation for plants across the field using the guidelines in Figure 2. Assess beetle populations, along with other defoliating pests, by either directly observing beetles or using a sweep net. If using a sweep net, take a 20-sweep sample in each area sampled to determine whether pests are still actively feeding in soybean.

soybean defoliation scouting plan
Figure 2. Soybean defoliation scouting plan.

Late-season – estimate pod injury and insect density: Randomly select two plants in five areas of the field. Count the number of total pods per plant and the number with bean leaf beetle damage (do not confuse with grasshopper damage, which usually extends through the pod to the seed). Take note of any clipped pods and whether beetles are actively feeding. Also, take a 20-sweep sample in each area to estimate bean leaf beetles per sweep. Refer to Table 2 for guidance on economic thresholds for late-season soybean sampling.

Table 2. Economic threshold (beetles/sweep) for late-season scouting (R5-R7). Table modified from University of Illinois and Purdue Extension.

Pod Injury Level Number of beetles/sweep in 30-inch rows
Less than 4 4-7 More than 7
0-8% Discontinue sampling Re-sample in 5 days Preventative control if pods are green
8-12% Re-sample in 5 days Control if pods are green Control if pods are green to yellow
Over 12% Control if pods are green and beetles are present Control unless pods are completely dry Control unless pods are completely dry

Scouting tips! Bean leaf beetles are easily disturbed and will drop from plants and seek shelter. They also seek shelter during warm parts of the day in cooler areas such as cracks in the soil or under residue. Move carefully through the field and consider scouting earlier or later in the day when it is cooler.

Management

The primary management concern is for food-grade soybeans or soybeans grown for seed where seed quality reductions from bean pod mottle virus are a significant concern. However, high populations of bean leaf beetles early in the season can severely defoliate seedling plants and lead to high first- and second-generation beetle numbers. Management of high early-season populations may be necessary to limit plant injury or reduce the number of beetles in subsequent generations.

Predicting populations: Bean leaf beetle populations essentially have boom and bust cycles. Cold winters are thought to keep beetle abundance in check. Every spring, a prediction for overwintering mortality is published in ICM News.

Tracking development: A degree-day model has been developed for monitoring bean leaf beetle. First-generation adults are estimated to emerge after 1,212 degree days (base 46°F) since egg-laying. Overwintered female beetles usually begin to lay their eggs after colonizing the bean fields. The degree-day estimation for the first-generation adults is calculated by accumulating the temperature beginning the week of soybean emergence. For adult beetles use the following:

  • Determine what week your soybean plants emerged from the soil.
  • Sample your soybean fields with a sweep net one week after the predicted peak emergence (1,212 GDD). If the number of beetles reaches or exceeds the threshold (Table 2), stop sampling. If the sample is below the threshold, sample the following week. If the sample remains below the threshold, sample a third and final week. If the threshold is not reached, an economic infestation of bean leaf beetles should not occur later in the season.
  • If the first-generation population is above the threshold, do not spray now, but scout the fields again in late August to monitor for the first-emerging beetles of the second generation. Based on the population size of the first generation, it is expected that the second generation will exceed the economic threshold. When the first beetles appear, spray the field with an insecticide (45-day preharvest interval or less).

Cultural control: Planting date may or may not have a consistent effect on bean pod mottle virus incidence, although late planting can protect pods from damage. However, it seems likely that the risk of an economically important level of infection is higher for the first-emerging soybean fields in an area.

Chemical control: Insecticidal seed treatments should be considered for early-planted fields or fields with a history of high bean leaf beetle populations or bean pod mottle virus. Using labeled rates of foliar insecticides can be used to manage defoliation and pod injury caused by bean leaf beetle adults. Use the scouting information in the previous section and the associated thresholds to help make treatment decisions. When estimating defoliation, the threshold is 30% defoliation for vegetative plants (prior to R1) and 20% for reproductive plants (R1 or later). Always scout fields after an insecticide application to ensure proper knockdown was achieved and to investigate whether recolonization occurred.

Currently no thresholds are available for managing bean pod mottle virus; however, insecticidal control has shown potential for managing this virus by reducing vector abundance. It may be wise to use lower thresholds in soybean seed fields or food-grade soybean fields. One concept is to target two pyrethroid sprays at overwintering beetles in late May or early June and at the presence of first-generation beetles in July. The idea behind this management tactic is to reduce the initial inoculation of the field and the mid-season inoculation and spread of the virus. An ISU study showed that under high vector abundance, seed treatments alone reduced bean pod mottle virus incidence; yield was only improved with the application of a mid-season insecticide application program. This approach is unlikely to be economical in fields that are not high risk for bean leaf beetle or bean pod mottle virus.

Originally prepared by Jeff Bradshaw and Marlin Rice. Updated by Erin Hodgson in 2017. Updated again by Ashley and Erin in 2022.

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